Method of suppressing bimetallic couple corrosion of magnesium metal articles



July 30, 1963 1 1 K. DE LON 3,099,083

METHOD OF S RE NG BIMETALLI OUPLE CORROSION MAGNESIUM METAL ARTICLES Filed Feb. 27, 1958 BY W -MQ United States Patent Oil ice 3" Patented July 30, 1%63 PETHUD OF EiUllRllfiHlG BEMETALLIC CGUPLE CGRROSHEN F MAGNESUM METAL ARTICLES Herbert K. De Long, Midland, Mich assignor to The Eow Chemical Company, Midland, Mich a corpora- .tion of Delaware Filed Feb. 27, 1958, Ser. No. 718,627 19 Claims. (Cl. 29527) This invention relates to an improved method of suppressing galvanic corrosion of articles formed of magnesium and magnesium 'base alloys (hereinafter referred to as magnesium metal) when electrically coupled by or to articles formed of a more noble metal and the asse r.- bly exposed to a corrosive environment.

Heretofore various means have been employed to minimize the galvanic corrosion of magnesium metal articles coupled by or to poorly compatible metal articles in an assembly exposed to corrosive conditions. As barriers between magnesium metal articles and poorly compatible metal articles, paint coatings and organic films are genrally protective under mild conditions but are not suitable for use in highly stressed joints nor at elevated temperatures at which such films detorm or decompose.

Neither entirely satisfactory are previously used metallic coatings consisting of cadmium or Zinc and usually applied to the poorly compatible, i.e., more noble, metal article rather than to the magnesium metal article in contact therewith. Cadmium and zinc, which are grouped close to magnesium in the electromotive force series of the elements, exhibit slightly smaller solution potentials than magnesium and would thus be expected to cause rela tively mild galvanic attack on magnesium when electrically coupled thereto. However cadmium coatings, to a greater extent zinc coatings, applied to articles incorporated in magnesium metal structures, are not particularly resistant to the alkaline corrosion products formed during galvanic action at the cathode, or more noble metal surface, so that under corrosive conditions these coatings are rather readily dissolved leaving t e adjacent, or underlying, magnesium metal article, as the case may be, open to more severe galvanic attack. Also readily dissolved from the surface of cadmium and zinc coatings by alkaline solutions are films normally formed on the more noble metal electrode during galvanic attack.

it has now been found that by interposing a substantially continuous barrier of tin or its alloys with cadmium and zinc between a magnesium metal article and each article of poorly compatible metal coupled thereby or thereto bimetallic couple corrosion of the magnesium metal article is effectively suppressed.

Tin-zinc alloys which are useful are t se comprising up to 60 percent zinc, the balance tin, though the alloys comprising tin and to 30 percent zinc are to be pro fer-red. The tin-zinc alloy comprising about percent zinc, the balance tin is believed to be the best compromise for many applications. Sim'larly, the tin-cadmium alloys comprising up to 60 percent cadmium, the balance tin, have been found useful in the practice of my invention though the alloys comprising tin and 15 to 30 percent cadmium are to be preferred. While tin is to be preferred for superior corrosion resistance cadmium or zinc alloys with tin have slightly less corrosion resistance but better physical properties.

Tin and the above defined tin-cadmium alloys and tinzinc alloys for purposes of the specification and claims are hereinafter referred to as tin metal.

The protection of magnesium metal from galvanic corrosion as provided by tin metal is better than that provided 'by cadmium or zinc alone even though tin, of these three elements, is the further removed from magnesium in the electrornotive force series. The superiority of tin metal is apparently due to a much greater stability of the film formed on the tin during galvanic cell action while coupled to magnesium metal. Thus the film of the oathode reaction product (probably tin hydroxide or possibly magnesium stannate) being a poor electrical conductor tends to retard current flow and 'by so doing suppresses the amount of magnesium going into solution as magnesium oxychloride or precipitating as magnesium hydroxide.

The invention then consists of the aforesaid method of suppressing bimetallic or galvanic corrosion of magnesium metal articles, the method bein herein fully described and particularly pointed out in the claims, the appended drawing illustrating several types of tin metal coated articles used in the practice of the invention in addition to assemblies of dissimilar metal parts illustrating the practice of the invention.

in carrying out the invention a magnesium metal article may be protected from severe galvanic attack in one or more of the following ways: (1) by providing a coating consisting of tin metal, (a) for the magnesium metal article, (12) for each more noble metal article in contact therewith, (2) by providing a coating of tin metal as an initial deposit or substrate prior to the application of a coating of cadmium, zinc, chromium or silver onto (a) the magnesium metal article, (17) each more noble metal article in contact with the said magnesium metal article, or (3) by interposing between the magnesium metal article and each more noble metal article in contact therewith separating means, such as a shim, comprising (a) tin metal, (b) a core metal provided with a coating of tin metal, or (c) a core metal provided with a composite coating comprising a substrate coating of tin metal overlaid with a surface coating of cadmium, zinc, chromium or silver.

The tin coating in each instance may be applied to articles poorly compatible with magnesium metal by well known methods such as electroplating, hot dipping, metal spraying, and immersion deposition. Magnesium metal articles may be tin coated, as by electroplating. Conventional electroplating methods of depositing tin metal may be used after a pretreatment of magnesium metal articles as described pp. 88 of the copyrighted manual, Magnesium Finishing, 1955 edition, published by The Dow Chemical Company. A single coating of tin 0.0001 to 0.001 inch thick is generally adequate though a thicker coating may be applied.

For composite coatings a substrate coating of tin about 0.0001 inch thick is adequate though a heavier coat may be applied. The surface or finish coating is preferably a regular deposit of cadmium, zinc, chromium or silver applied by conventional methods and about 0.0002 to 0.0005 inch thick. Finish coatings of Zinc or chromium generally show the best corrosion resistance.

The composite coating described provides corrosion protection superior to that provided by a coating of equivalent thickness of cadmium, zinc, chromium or silver alone. These composite coatings are important for high temperature applications in which certain magnesium alloys are used, e.g., in the range of 400-700 F., since unalloyed tin melts at approximately 450 F. whereas cadmium melts at approximately 610 F., zinc at appr ximately 785 F, chromium at approximately 2940 F, and silver at approximately 1760 F. These metals, each of which thus exhibit a higher melting temperature than tin, are also metals normally resistant to chemical corrosion. Subsequent heating of the composite coating during use at elevated temperatures is believed to cause alloying of the tin substrate with the overlaid coating. This does not minimize the efiectiveness of the tin in increasing the corrosion resistance of the surface metal coating, and indeed, a desirable result is achieved in that the alloys produced exhibit higher melting temperatures and better physical properties than does pure tin and are thus more suitable for high temperature applications. The composite coating may also be pretreated before use, if desired, by heating to cause alloying of the substrate, though complete melting of the surface metal coating is to be avoided.

The same methods described above for the application of coatings of tin are generally useful for the application of tin-cadmium and tin-zinc alloys upon selecting appropriate compositions to achieve, by the method employed, the desired composition of the deposited coating. For example, a coating of tin-zinc may be applied by electroplating according to well-known methods; however, the ratio of tin to zinc deposited from the bath is not the same as the tin to zinc ratio in the plating bath. Surface coating thicknesses of 0.0001 to 0.001 inch may be used though a thickness of about 0.0005 inch is to be preferred. A substrate coating of the said alloys at least 0.0001 inch thick is adequate.

The aforementioned separating means, of which a shim is an example, may be formed of a tin metal. In order to have desirable mechanical strength the separating means are best formed of heavier gauge metal than that corresponding to the thickness of the herein described coatings. Shims are preferably 0.001 to 0.01 inch thick though heavier ones may be used. Separating means formed of a core metal provided with a coating of tin metal or with a substrate coating of tin metal and a surface coating of cadmium, zinc, chromium or silver may be used if desired. The same coating methods and thicknesses and alloy compositions described earlier in this specification would apply to the preparation of coated separating means of this type.

Referring to the said drawing,

FIGS. 1, 2, 3 and 4 illustrate several types of coated articles used in the practice of the invention.

FIG. 5 illustrates the use of -a tin metal shim and a tin metal coated fastener in an assembly according to one mode of practicing the invention.

FIGS. 6 and 7 illustrate other forms of bolts which may be used having double and single coatings respectively, FIG. 7 showing also a test assembly of bolt, washer, nut and panel.

FIG. 1 shows a View of a coated article, such as a structural panel, comprising a metal core 10 which may be magnesium metal, or a more noble metal, such as steel, provided on its top and bottom surfaces 11, 12 with a corrosion resistant coating of tin metal. As shown the article is representative both of a protected magnesium metal article and of a nominally poorly compatible metal anticle made galvanically compatible with magnesium metal.

In FIG. 2, in transverse cross section, is shown a coated article such as a structural member in the shape of an angle comprising a metal core 20 which may be of magnesium metal, :or a more noble metal, such as steel, provided on all surfaces 21 with a composite coating consisting of a substrate coating 22 of tin metal and a surface coating 23 of cadmium, zinc, chromium or silver.

FIG. 3 is an exploded side elevational view, in section, of an assembly comprising a bolt 30', lockwasher 31, and nut '32, each member comprising, respectively, a core 33, 34, 35 formed of a metal poorly compatible with magnesium metal, such as steel, and provided with a continuous coating 36, 37, 38, of tin metal. If desired, the cores 33, 34, 35 may be formed from magnesium metal and provided with the coatings 36, 37, 38 described. Normally the cores of fasteners are formed of a metal such as steel having good physical properties.

FIG. 4 is an exploded side elevational view in section, of a bolt 40 and nut 41 assembly both members comprising respectively a core 42, 43 formed of a metal poorly compatible with magnesium metal, a substrate coating 44, 45 of tin metal and a surface coating 46, 47

4 of cadmium, zinc, chromium or silver. If desired, the core 32, 43 of both members 40, 41 may be formed of a magnesium metal.

FIG. 5 is a transverse cross section through part of a composite structure having a magnesium metal part 50, such as a structural panel, electrically coupled at the faying surfaces to -a more noble metal part 51, such as an angle, the two parts being mechanically connected by a fastener such as a bolt 52 and a nut 53. (As used in the specification and claims, the term faying surface refers to a surface of a metal article in contact with another metal article.) Bolt 52 a comprises a core 54, formed of a metal poorly compatible with magnesium such as steel, provided with a coating 55 of tin metal. Interposed between the faying surfaces of pants 50 and 51 is a shim 56 formed of tin metal. Shim 56 prevents contact of magnesium metal part 50 and metal part 51 which is poorly compatible therewith and thus suppresses bimetallic corrosion of part 50. It is not necessary to coat the nut 53 if the parts are assembled as shown. Coating 55 on the fastener bolt 52 prevents contact of the poorly compatible metal core 54 with magnesium metal part 50.

The effectiveness of the use of tin as .a surface coating and as a substrate coating according to this invention was tested in the following way:

A test panel (No. 1) in. thick formed of the magnesium alloy AZ31B having a nominal composition of 3 percent aluminum, 0.2 percent manganese, 1 percent zinc, the balance magnesium was given an anodized finish sometimes referred to as Dow No. 17 and fully described in Metal Finishing Guidebook Directory, 26th edition (1958), pp. 5167, and then sealed by spraying on a very low solids air drying clear vinyl lacquer. This technique general corrosion so that galvanic corrosion only can be studied. Rows of bolt holes, three holes per row, were then drilled and countersunk on 1 /2 in. centers through the panel to accommodate in. x 1 in. flat head mild steel bolts (either slot or Phillips type head). Groups of three A in. x 1 in. flat head steel bolts as illustrated in FIG. 6 were given the various metal coatings to be evaluated. The bolts so prepared were then assembled on the above described panel, three bolts of the same coating type per row, each with a split lock washer and nut, the washer being adjacent to the nut, as illustrated in FIG. 7. Electrical contact between each bolt and the panel was checked and assured, before exposure treatment, with a resistance meter. Each of the three bolts, nuts, and washers of each group prior to assembly in the test panel was coated as follows:

Test group No. 1, row 1-This was electroplated with a 0.0005 inch coating of metallic tin in a conventional manner using a standard alkaline stannate type tin plating solution.

Blank group No. 1, row 2This was electroplated with a 0.0005 inch coating of metallic zinc from a conventional zinc cyanide type bath.

Blank group No. 2, row 3This was electroplated with a 0.0005 inch coating of metallic cadmium from a conventional cadmium cyanide bath.

Test group No. 2, row 4-This had received a 0.0001 inch substrate coating of metallic tin electroplated from a standard alkaline stannate type tin plating solution, and a surface coating of metallic zinc 0.0004 inch thick electroplated from a conventional Zinc cyanide bath.

Blank group No. 3, row 5This group was bare of any coating and when assembled on the panel was used as a reference standard to establish zero ratings.

The test panel (No. 1) containing the 5 rows of bolt assemblies was then exposed to: 20 percent aqueous sodium chloride solution (salt spray) fog for 200 hours. This exposure treatment was made in accordance with A.S.T.M. specification B117-54t. After the exposure period the test panel was washed free of residual corrosion products and salts and each bolt row then examined for the extent of corrosion around the bolt areas. The extent of corrosion in each row area so observed was estimated and the estimates arranged in order of magnitude on a l0 linear scale, 0 representing complete failure to suppress corrosion of the magnesium test panel and 10 the absence of corrosion, the intermediate values representing corresponding intermediate amounts of corrosion. The results so obtained for each row area are listed in the following table.

The values listed in Table 1 clearly show that upon applying either a tin surface coating or a tin substrate coating to a steel fastener, the tendency for galvanic corrosion to occur to a magnesium metal article, coupled by said fastener, is suppressed.

An additional test series to show the efiect of expo-sure of a complete test assembly to an elevated temperature was carried out as follows: A test panel (No. 2) A in. thick formed of the magnesium alloy AZ3lB was given the Dow No. 17 anodized finish referred to above, and then sealedby spraying on a very low solids air drying clear vinyl lacquer. Seven rows of bolt holes, three holes per row, were then drilled and countersunk on 1 /2 in. centers through the panel to accommodate in. X 1 in. fiat head mild steel bolts. Groups of three /4 in. X l in. mild steel bolts and complementary nuts and washers were given various metal coatings to be evaluated and each bolt so coated was inserted through the panel fastened in place with washer and nut in rows of three each and checked for electrical contact as described in the first test series. The three bolts, nuts, and washers of each group were coated as follows:

Test group No. 3, row l-This was electroplated with a 0.0001 in. substrate coating of metallic tin using a standard alkaline stannate type tin plating solution. -A surface coating of 0.0004 in. cadmium electroplate was then applied from a conventional cadmium cyanide bath.

Test group No. 4, row 2-This was electroplated with a 0.0002 in. substrate coating of metallic tin using a standard alkaline 'stannate tin plating solution. A surface coat'mg of 0.0003 in. zinc electroplat-e was then applied from a conventional zinc cyanide bath.

Test group No. 5, row 3-This was electroplated with a 0.0003 in. substrate coating of metallic tin using a standard alkaline stannate tin plating solution. A surface coating of 0.0002 in. ch omium electroplate was then applied from a conventional chromic acid bath.

Test group No. 6, row 4-Tnis was electroplated with a 0.0002 in. substrate coating of metallic tin using a standard alkaline stannate tin solution. A surface coating of 0.0003 in. silver electroplate was then applied from a conventional silver cyanide bath.

Blank group No. 4, row 5This was electroplated with a 0.0003 in. coating of chromium using a conventional chromic acid bath.

Blank group No. 5, row 6This was electroplated with a 0.0005 in. coating of silver using a conventional silver cyanide bath.

Blank group No. 6, row 7This group was bare of any coating.

The panel containing the described groups of bolt and nut assemblies was then exposed to a temperature of 700 F. for 30 minutes following which, after cooling to room temperature, it was exposed to a 2.0 percent aqueous sodium chloride solution (salt spray) fogin accordance with the test known as A.S.T.M. specification B-l17-54t. After 50 hours exposure to the test the panel was washed Table 2 Rating-corrosion Panel of magnesium panel N o. 2, Group Coating on fasteners Bolt row N0. 50 hrs. 200 hrs. exposure exposure 1 Test No. 3 .0001 in. tin plate -I 5 -5 .0004 in. cadmium plate. 2 Test No. 4-.-- .0002 in. tin plate 8 6 .0003 in. zinc plate. Test No. 5..-- .0003 in. tin plate I- 7 7 .0002 in. chromium plate. 4 Test No. 6-.-- .0002 in. tin plate 8 8 .0003 in. silver plate. 5 Blank N0. 4..- .0003 in. chromium 5 5 plate. 6 Blank N o. 5... .0005 in. silver plate... 7 7 7 Blank No. 6... none 0 0 The values in Table 2 show that the use of a tin substrate coating causes increased suppression of galvanic corrosion of magnesium metal even after exposure of an assembly to an elevated temperature.

Still an additional test series was carried out as follows:

Three test panels (Nos. 3, 4, 5) each 4 in. X 6 in. X /21 in., formed of the magnesium alloy AZ31B, were given the Dow No. 17 anodized finish, referred to above in the first test series, and then semed by spraying on a very low solids air drying clear vinyl lacquer. A row of three bolt holes was then drilled on 1 /2 in. centers through each panel to accommodate A in. diameter bolts.

Blank group No. 7A row of three bolt holes to accommodate A bolts was then drilled on 1 /2 in. centers through a mild steel bracket, having a transverse cross section in the shape of an angle, 1 /4 in. X 6 in. The bracket was then placed on test panel N0. 3 so that the holes aligned and A in. X l in. mild steel stove bolts were inserted through the respective holes, the head of each bolt being on the side of the test panel opposite the bracket. A mild steel split lock washer and nut was assembled on each bolt and tightened.

Test group No. 7A second mild steel bracket was prepared in the same manner as the first and attached to test panel No. 4 in a similar way except that a shim, 1 /4 in. X 6 in. X .036 in., formed of tin, and having three A in. bolt holes formed therethrough on 1 /2 in. centers, was interposed between the test panel and the bracket, so as to .be aligned with thebracket, and the parts attached with A; in. X 1 in. mild steel stove bolts in a way similar to that shown in FIG. 5 of the drawing. The shim, being wider, extended about in. beyond either side of the bracket.

Test group No. 8-A third mild steel bracket was prepared in the same manner as the first bracket described above and then a 0.0005 in. coating of metallic tin was applied to it by electroplating from a standard alkaline stannate tin plating solution. The tin plated bracket was then attached to test panel No. 6 with A in. X l in. mild steel stove bolts in the same manner as described for blank group No. 7. a

The described three assemblies were exposed to a 20 percent aqueous sodium chloride solution (salt spray) fog in accordance with the test known as ASTM specification B-l l7-54t. The panels were exposed with the side be ing the brackets disposed upwardly. After periods, respectively, of 72, 120, and 250 hours the test was interrupted and each panel was washed free of residual corrosion products and salts and examined visually. The extent of a corrosion of each panel adjacent the attached bracket was rated on a O to scale in a manner similar to that described for the first test series, the extent of corrosion of the panel adjacent the bracket of blank group No. 7 being rated zero on this scale after each test period. Ratings for each group, or panel, are listed in Table 3.

-Magnes=ium metal structures which are to be exposed to a corrosive environment are generally protected as by painting. Magnesium metal structures protected by the method of my invention possess the additional advantage that paint coatings, properly selected and applied to them, adhere unusually well and extend the period of effective protection from corrosion.

What is claimed is:

1. An improved method of suppressing galvanic corrosion of a magnesium metal article in a composite metal structure comprising a magnesium metal article and a ferrous metal article on being mechanically and electrically connected and subjected to the electrolytic action of the same environment, which comprises interposing between the articles a continuous layer of a tin metal selected from the group consisting of tin, tin-zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin, whereby the current generated by the one article acting as an anode to the other passes through the said layer of tin metal and said layer of tin metal presenting an unbonded surface to at least one of said metal articles.

2. An improved method of suppressing galvanic corrosion of a magnesium metal article mechanically coupled by at least one ferrous metal fastener comprising interposing a continuous barrier layer of a tin metal selected from the group consisting of tin, tin-zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin, between the contacting surfaces of said article and said fastener said layer of tin metal presenting an unbonded surface to at least one of said contacting surfaces.

3. The method as in claim 2 in which the tin metal is nnalloyed tin.

4. The method as in claim 2 in which the tin metal is a tin-zinc alloy comprising up to '60 percent zinc the balance tin.

5. The method as in claim 4 in which the tin-zinc alloy comprises to percent zinc the balance tin.

6. The method as in claim 2 in which the continuous barrier layer is applied to the ferrous metal fastener.

7. The method as in claim 2 in which the continuous barrier layer is applied to the magnesium metal article.

8. An improved method of suppressing galvanic corrosion of a magnesium metal article electrically coupled at faying surfaces to at least one ferrous metal article comprising interposing a continuous barrier layer of a tin metal selected from the group consisting of tin, tinzinc binary alloys containing at least percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin, between the faying surfaces of the articles said layer of tin metal presenting an unbonded surface 'to at least one of said faying surfaces.

9. The method as in claim 8 in which the continuous barrier layer comprises discrete separating means, said separating means being formed of a tin metal selected from the group consisting of tin, tin-zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin.

10. The method as in claim 9 in which the separating means comprises a metal core provided with a continuous coating of a tin metal selected from the group consisting of tin, tin-Zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin.

11. An improved method of suppressing galvanic corrosion of a magnesium metal article mechanically coupled by at least one ferrous metal fastener comprising applying a continuous composite metal coating to at least one of the contacting surfaces therebetween and said continuous composite metal coating presenting an unbonded surface to at least one of said contacting surfaces, said composite metal coating comprising a continuous substrate coating of a tin metal selected from the group consisting of tin, tin-zinc binary alloys containing at least 40 percentof tin, and tin-cadmium binary alloys containing at least 40 percent of tin, said coating of tin metal being overlaid with a continuous surface coating of a metal selected from the group consisting of cadmium, zinc, chromium, silver said composite metal coating being transformable, at least in part, into a tin alloy upon being subjected to an elevated temperature above about the melting temperature of tin.

12. The method as in claim 11 in which the continuous composite metal coating is applied to the galvanically incompatible metal fastener.

13. The method as in claim 11 in which the tin metal substrate comprises unalloyed tin and the surface coating is ZlIlC.

14. The method as in claim 11 in which the tin metal substrate comprises 15-30 percent zinc, the balance tin.

15. An improved method of suppressing galvanic corrosion of a magnesium metal article electrically coupled at faying surfaces to at least one ferrous metal article comprising applying a continuous composite metal coating to at least one of the faying surfaces and said layer of tin metal presenting an unbonded surface to at least one of said faying surfaces, said composite metal coating comprising a continuous substrate coating of a tin metal selected from the group consisting of .tin, tin-Zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin, said coating of tin metal being overlaid with a continuous surface coating of a metal selected from the group consisting of cadmium, zinc, chromium, silver said composite metal coating being transformable, at least in part, into a tin alloy upon being subjected to an elevated temperature above about the melting temperature of tin.

16. The method as in claim 15 in which the continuous composite metal coating is applied to the ferrous metal article.

17. The method as in claim 15 in which the continuous composite metal coating is applied to the magnesium metal article.

18. An improved method of suppressing galvanic corrosion of a magnesium metal article electrically coupled at faying surfaces to at least one ferrous metal article comprising interposing separating means in the form of a shim between the faying surfaces, said shim comprising a core metal provided with a continuous composite coating comprising a continuous substrate coating of a tin metal selected from the group consisting of tin, tin-zinc binary alloys containing at least 40 percent of tin, and tin-cadmium binary alloys containing at least 40 percent of tin, and a continuous surface coating of a metal selected from the group consisting of cadmium, zinc, chromium, silver said composite metal coating being transformable, at least in part, into a tin alloy upon being subjected to an elevated temperature above about the melting temperature of tin.

19. An improved method of suppressing galvanic corrosion of a magnesium metal article electrically coupled at faying surfaces to at least one ferrous metal article comprising applying a continuous composite metal coating to at least one of the faying surfaces and said continuous composite metal coating presenting an unbonded surface to at least one of said faying surfaces, said composite metal coating comprising a continuous substrate coating of metallic tin overlaid with a continuous surface coating of a metal selected from the group consisting of cadmium, zinc, chromium, silver, and heating said composite coating to an elevated temperature below about 650 F. whereby the metallic tin substrate forms an alloy with at least a portion of the overlaid coating, said alloy consisting of at least 40 percent of tin.

References fitted in the file of this patent UN ITED STATES PATENTS Nielsen Nov. 3, Basch May 12, Smith Dec. 29, Cook Feb. 20, Rutherford Apr. 29, Fergus Nov. 1, Nachtman May 12, Brooks et al Feb. 21, Sever Nov. 19,

OTHER REFERENCES Ser. No. 383,003, Hilpert (A.P.C.), published May 18, 1943. This A.P.C. application is abandoned. 

1. AN IMPROVED METHOD OF SUPPRESSING GALVANIC CORROSION OF A MAGNESIUM METAL ARTICLE IN A COMPOSITE METAL STRUCTURE COMPRISING A MAGNESIUM METAL ARTICLE AND A FERROUS METAL ARTICLE ON BEING MECHANICALLY AND ELECTRICALLY CONNECTED AND SUBJECTED TO THE ELECTOLYTIC ACTION OF THE SAME ENVIRONMENT, WHICH COMPRISES INTERPOSING BETWEEN THE ARTICLES A CONTINUOUS LAYER OF A TIN METAL SELECTED FROM THE GROUP CONSISTING OF TIN, AND TIN-CADALLOYS CONTAINING AT LEAST 40 PERCENT OF TIN, AND TIN-CADMIUM BINARY ALLOYS CONTAINING AT LEAST 40 PERCENT OF TIN, WHEREBY THE CURRENT GENERATED BY THE ONE ARTICLE ACTING AS AN ANODE TO THE OTHER PASSES THROUGH THE SAID LAYER OF TIN METAL AND SAID LAYER OF TIN METAL PRESENTING AN UNBONDED SURFACE TO AT LEAST ONE OF SaID METAL ARTICLES. 